1. Field of the Invention
The systems and methods of this invention relate to pacing treatment of the heart by means of an implantable device, more specifically to systems and methods for providing such pacing without the use of conventional lead/electrode systems by transmitting pacing energy acoustically from an implanted or externally located transmitter to an implanted receiver-stimulator. Still more specifically, the present invention provides methods to improve the pacing pulse shape and efficiencies of such a pacemaker system.
In conventional pacemaker systems that apply an electrical pulse to the heart through a lead wire terminated at an electrode structure, considerable effort has been expended to optimize the shape of the electrical pacing pulse. This work has been driven primarily by two factors: 1) the constant need to optimize energy efficiency in the pacemaker to obtain maximum battery life, and/or minimize the size of the implanted device; and 2) the need in all modern conventional (wired) pacemakers to use the stimulating electrode to immediately sense, following stimulation, the electrogram signal from the cardiac tissue.
An early cardiac pacemaker patent (U.S. Pat. No. 3,057,356 by Greatbatch) shows, in one embodiment, the use of a storage capacitor between the output circuitry and the electrode attached to the heart, the capacitor providing the pulse generator with a lower output impedance and thus enabling a higher initial current to the electrode as compared to a directly-coupled output circuit. Most modern pacemaker designs still use a coupling/storage capacitor in the output circuit.
One undesirable effect of applying a pacing stimulus to cardiac tissue is electrode polarization, or generation of an after-potential, at the electrode-tissue interface. Depending on the amplitude of the stimulus, the after-potential can remain at hundreds of millivolts immediately after the stimulus, decaying to zero over the course of tens of milliseconds. In modern pacemakers, the stimulation electrode is also typically used to detect the electrogram of the heart; therefore, the presence of an after-potential may inhibit detection of an evoked response immediately after stimulation. The use of a capacitively-coupled output circuit alone reduces the after-potential by lowering the average DC component of the pacing waveform. However, in most cases, additional techniques are required to enable the measurement of evoked potentials following pacemaker stimulation. Methods to accomplish this have included applying a bi-phasic waveform (U.S. Pat. No. 3,835,865 by Bowers), or by applying more complex stimulation waveforms (U.S. Pat. No. 4,343,312 by Cals et al., and U.S. Pat. No. 4,373,531 by Wiitkampf et al.), or by discharging the polarization potential after the stimulus pulse (U.S. Pat. No. 4,399,818 by Money, U.S. Pat. No. 4,498,478 by Bourgeois, and U.S. Pat. No. 5,843,136 Zhu et al.). Other methods have been disclosed to further control and optimize the shape of the pacemaker stimulation waveform to produce a more energy efficient system (U.S. Pat. No. 5,782,880 by Lahtinen et al.).
As described in co-pending provisional application Ser. No. 11/315,523, and Ser. No. 11/315,524, a method of cardiac and other tissue stimulation uses one or more implantable acoustic receiver-stimulators for cardiac or other tissue stimulation, along with an implanted or externally-applied acoustic controller-transmitter. The implanted receiver-stimulator device comprises a piezoelectric sensor component, which passes an alternating current signal representing the acoustic field impinging upon it to a rectifier/filter circuit. The circuit functions to convert the AC signal from the piezoelectric sensor to a substantially DC waveform, the waveform being present for as long as the piezoelectric sensor is within an acoustic driving field, and importantly, the waveform produced essentially duplicates the shape of the envelope—contour of the peaks and valleys—of the transmitted acoustic energy. The rectifier/filter circuit might typically comprise a half wave rectifier, a full wave rectifier, or a voltage doubler circuit, to recite just a few possible implementations, followed by a filter comprising a series inductor or a parallel capacitor or combinations thereof, again to recite just a few possible implementations. Lastly, the tissue contacting electrodes might be typical of the current state-of-the-art in implantable electrode design and materials.
The transmitted acoustic energy is produced either by an acoustic controller-transmitter implanted beneath the skin of the subject and powered by a battery, or an externally positioned transducer controlled and powered by external means. For an implanted acoustic controller-transmitter, clinical utility requires that the device have a reasonable size (typical of current pacemakers) and that the device function without recharging for a period in excess of three years, preferably seven years. Such a temporal performance requirement places significant demands on the overall efficiency of the system. These demands are not unlike those faced by the first and all subsequent conventional pacemaker systems, which have been addressed by many improvements since the introduction of implantable pacemakers. The methods described herein directly attend to the demands of energy efficiency in a wireless pacemaker system utilizing acoustic energy and signal transmission, and also allow significant reduction in the physical size of such a device.
The issue of sensing an intrinsic or evoked electrogram through the same electrode used for stimulation is resolved by the co-pending applications mentioned above, which describe a system where the electrogram is sensed remotely from the stimulating electrode by means incorporated into the acoustic controller-transmitter device. More advanced versions of the receiver-stimulator could incorporate means to sense the electrogram and either process the information directly or transmit it back to the controller-transmitter through acoustic or other transmission methods. With such advancements, the process of sensing the electrogram would benefit from the stimulation waveform control produced by the methods describe in this disclosure.
2. Background Art    U.S. Pat. No. 3,057,356, W. Greatbatch, Medical Cardiac Pacemaker, October 1962.    U.S. Pat. No. 3,835,865, D. L. Bowers, Body Organ Stimulator, September 1974.    U.S. Pat. No. 4,343,312, G. Cals et al., Pacemaker Output Circuit, August 1982.    U.S. Pat. No. 4,373,531, Wittkampf et al., Apparatus for Physiological Stimulation and Detection of Evoked Response, February 1983.    U.S. Pat. No. 4,399,818, Money, Direct-Coupled Output Stage for Rapid-Signal Biological Stimulator, August 1983.    U.S. Pat. No. 4,498,478, Bourgeois, Apparatus for Reducing Polarization Potentials in a Pacemaker, February 1985.    U.S. Pat. No. 5,782,880, Lahtinen et al., Low Energy Pacing Pulse Waveform for Implantable Pacemaker, July 1998.    U.S. Pat. No. 5,843,136, Zhu et al., Pacing Output Circuitry for Automatic Capture Threshold Detection in Cardiac Pacing Systems, December 1998.    Saksena and Goldschlager: “Electrical Therapy for Cardiac Arrhythmias-Pacing, Antitachycardia Devices, Catheter Ablation”, W.B. Saunders Co, Philadelphia, 1990.    O. Soykan: Automated Piecewise Linear Modeling of Pacing Leads, Medtronic Inc., Fridley, Minn., 1994.